Technical Field
The present disclosure relates to a microelectromechanical gyroscope with compensation of quadrature signal components and to a method for controlling a microelectromechanical gyroscope.
Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) is increasingly widespread in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one mass connected to a supporting body (also referred to as “stator”) by springs and movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective electrodes facing one another so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, so it is possible to trace back to the relative displacement of the movable mass with respect to the supporting body and hence to the force applied. Conversely, by supplying appropriate bias voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. Moreover, to obtain electromechanical oscillators the frequency response of the inertial MEMS structures is exploited, which typically is of the second-order low-pass type, with a resonance frequency.
MEMS gyroscopes, in particular, present a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and coupled together so as to have one translational or rotational relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation with controlled amplitude at the resonance frequency. The other mass is driven in oscillatory motion and, in the case of rotation of the microstructure with respect to a pre-set sensing axis with an angular speed, is subjected to a Coriolis force proportional to the angular speed itself. In practice, the driven mass operates as an accelerometer that makes it possible to detect the Coriolis force and acceleration and hence to trace back to the angular speed.
To operate properly, a MEMS gyroscope requires, in addition to the microstructure, a driving device, which has the task of keeping the movable mass in oscillation at the resonance frequency and with controlled amplitude, and a device for reading the displacements of the driven mass, in accordance with the degree of freedom of the driving mass. These displacements, in fact, indicate the Coriolis force and, consequently, the angular speed and may be detected through electrical reading signals correlated to the variations of the capacitive coupling between the driven mass and the stator. As a result of driving at the resonance frequency, the reading signals, determined by rotation of the gyroscope and correlated to the angular speed, are in the form of dual-side-band-suppressed-carrier (DSB-SC) signals; the carrier is in this case the velocity of oscillation of the driving mass and has a frequency equal to the mechanical resonance frequency.
Displacements of the driven mass is normally read by a charge-to-voltage converter, which supplies a signal proportional to the capacitance present between the driven mass itself and the supporting body. The signal generated by the charge-to-voltage converter is demodulated and filtered for extracting the modulating signal, which represents the angular speed of the supporting body about the sensing axis.
Since, however, the MEMS gyroscope has a complex structure and the electromechanical interactions between the movable masses and the stator are frequently nonlinear, the useful signal components are frequently superimposed on spurious components, which are not significant for measurement of the angular speed. The spurious components may be due to various causes. For example, causes of disturbance that are practically impossible to eliminate are the manufacturing imperfections and the process spread so that the behavior of real devices differs in a merely statistically foreseeable way from the design behavior. A very common defect depends upon the fact that the mass used for driving oscillates in a direction not perfectly coinciding with the degree of freedom envisaged in the design stage. In this case, the driving defect has an effect on the useful signal, introducing an unknown amplitude component at the same frequency of the carrier and phase-shifted by 90° (quadrature disturbance).
On the other hand, the contribution of the components of disturbance in many cases is significant and cannot be simply neglected, without introducing unacceptable distortions.
To solve the above problem, it has been proposed to use compensation signals taken from the driving device. The driving device is in fact coupled to the driving mass so as to form a resonant loop that defines the amplitude and frequency of the carrier. The resonant loop generates signals phase-shifted by 90° with respect to the carrier that are collected and supplied at input to the charge-to-voltage converter through a capacitive coupling. The latter, if appropriately sized, enables injection or subtraction of an amount of charge that varies in a sinusoidal way at the frequency of the carrier with a phase shift of 90° and compensation of the quadrature components of disturbance.
Albeit representing an improvement, the solution described still presents some limitations, especially due to the fact that the sizing of the capacitance for applying the compensation signal at input to the charge-to-voltage converter is difficult. The coupling capacitance determines, in fact, the amount of charge injected or subtracted, and, if it is not properly calibrated, the compensation is not complete. On the other hand, the degree of quadrature disturbance may vary in time, for example as a function of the temperature or on account of ageing, especially of the movable parts of the micromechanical structure. Compensation is hence not stable over time.